Annotation Usage in Kotlin

Introduction

Annotation Usage is a fundamental concept every Kotlin developer should understand. Annotations attach metadata to declarations. Frameworks like Spring, Retrofit, and Room read them to configure behaviour at compile or runtime.

Apply annotations before declaration using @AnnotationName. In this tutorial you will learn the syntax, walk through a complete example program, study the sample output, and review best practices so you can apply the concept confidently in your own projects.

Definition

• Apply annotations before declaration using @AnnotationName.
• Annotations can include constructor parameters.
• Used by frameworks for configuration and validation.

Syntax

@Deprecated("Use newApi")

Annotation Usage in Kotlin Example Program in Kotlin

@Deprecated("Use newMethod", ReplaceWith("newMethod()"))
fun oldMethod() = "old"

fun newMethod() = "new"

fun main(args: Array<String>) {
    println(newMethod())
}

Sample Output

new

When to use

Use annotations to declare framework configuration declaratively instead of wiring everything manually.

How it works

1. The program starts with a main function — the entry point that runs when you execute the file.

2. fun oldMethod() = "old" assigns or updates a value used later in the program.

3. fun newMethod() = "new" assigns or updates a value used later in the program.

4. The println(newMethod()) statement writes a line to the console — this produces part of the sample output below.

5. Apply annotations before declaration using @AnnotationName.

6. Run the program in IntelliJ IDEA, Android Studio, or with the Kotlin command-line compiler (kotlinc / kotlin). Compare your console output with the sample output shown below.

Best Practices

• Understand the core idea: apply annotations before declaration using @AnnotationName.
• Prefer readable names and small functions so examples map directly to real projects.
• Run and modify the example — change values and observe how the output changes.

Common Mistakes

• Skipping the example and only reading the definition — hands-on practice cements the concept.
• Copying syntax without understanding nullable vs non-nullable types or scope rules.
• Ignoring compiler warnings that often point to safer alternatives.

Key Points

• Apply annotations before declaration using @AnnotationName.
• Annotations can include constructor parameters.
• Used by frameworks for configuration and validation.
• Test the example locally and verify the output matches the sample.
• Experiment by changing input values to see how behaviour changes.

Notes

• Semicolons at the end of statements are optional in Kotlin.